Traditionally, pesto sauce meant a sauce made with basil, olive oil, pine nuts, Parmesan, and garlic. But these days you can use any leafy green (spinach, parsley, mint, whatever), as well as any sort of nut, such as walnuts, pistachios, or almonds. And while hard cheeses are best for making pesto, you can experiment with goat's milk cheeses, sheep's milk cheeses. and others.